What is the relationship between having full valence shells and formal charges?
The formal charge is the difference between the number of valence electrons "belonging" to the bonded atom and those in the full valence shell.
A quick formula for calculating the formal charge (FC) is
FC = V – L – B, where
V = number of valence electrons in isolated atom
L = number of lone-pair electrons
B = number of bonds
1. Let's apply this to the boron atom in BH₄⁻.
V = 3; L = 0; B = 4.
So FC = 3 – 0 – 4 = -1
B has a formal charge of -1 even though it has a full valence shell.
2. What about the C atom in CH₄?
V = 4; L = 0; B = 4.
So FC = 4 – 0 – 4 = 0
Here C has a full valence shell and a formal charge of 0.
3. Now look at the hydronium ion.
V = 6; L = 2; B = 3.
So FC = 6 – 2 – 3 = +1.
O has a full valence shell and a formal charge of +1.
In each case, the atom has a full valence shell but the formal charge can be negative, zero, or positive.